The first 'bifoiler' sailing boat


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This project produced what was, as far as I am aware, the first sailing hydrofoil with just two hydrofoils, (i.e. a 'bifoiler') one hydrofoil mounted ahead of the other. The idea has since been extensively and very successfully developed for the Moth dinghy class and also for some other small sailing boats. I dont think that Moth sailors could have copied my developments since these were not publicised at the time, rather I believe that it was the limits set by the Moth class rules that drove the innovators in that class to explore this concept.  I would say that in recent years the Moth sailors and others have developed the concept to a level of refinement far beyond my early experiments. 

First thoughts

Having been a member of the Amateur Yacht Research Society for some years, I was aware that the majority of hydrofoil sailing boats have three hydrofoils supporting the boat as a tripod, a logical approach. In most cases a widely spaced pair of hydrofoils is mounted forward and a single hydrofoil aft, but the reverse arrangement has also been tried and can work. There have also been configurations with four hydrofoils either positioned as the wheels of a car, or in a diamond pattern, ie a foil at each end and two lateral foils. 

Three or four widely spaced hydrofoils offers the potential for good stability to resist sail heeling moment, but it also implies a fairly expansive structure from which to mount those hydrofoils. Typically this structure is a multi hull sailing boat, either a catamaran or a trimaran. Since I was looking for a quick lightweight boat building project I was reluctant to head off in that direction and I wondered whether something simpler and more portable could be achieved by considering just two hydrofoils, one ahead of the other and mounted inline beneath a single hull. Since I was considering a small boat that would be lighter than most sailing dinghies, I reckoned that positioning my own body weight would give me the righting moment to carry sufficient sail, it is only with larger vessels having proportionately lighter crew weight that a broard plan form becomes necessary to couteract sail heeling moment.  Compared to other sailing hydrofoils, mine would be the equivalent of a bicycle as opposed to a tricycle or quadricycle. If you had never seen a bicycle you might well think it to be a totally impractical idea but in fact bicycles work rather well, better for most purposes than the more logical tricycles and quadricycles. That fact gave me some hope and lead me to embark on a somewhat crazy project.

So the initial concept was a single narrow hull with two hydrofoils below it, a sail mounted above and one or more crew members slung from a racing dinghy style trapeze wire with feet resting on the hull. Steering would by a tiller acting on one of the hydrofoils. Since I was initially thinking of a 'sailing bicycle' I first considered steering the forward rather than the rear hydrofoil, but I later decided to go with aft steering for mechanical simplicity. For the hull, I first imagined nothing more than a length of aluminium tube which would make the boat a 'sinker' in sailboard parlance. (a sinker is a sailboard having less buoyancy than the total weight of board and sailor, it can only operate by dynamically generated forces). Later I decided on a narrow box section plywood hull that would at least have buoyancy to support my weight plus some reserve.

So, I had an idea for a very portable and quite easy to build hydrofoil sailing boat but would it work?  Well, if you have watched an expert sail boarder water-start a 'sinker' sailboard there would seem to be a possibility that this hydrofoil boat could be sailed in the same way. You start with the wet-suited sailor lying in the water, feet in foot straps on the hull, the trapeze wire hooked to harness and the sail above him to windward. The wind catches the sail tensioning the trapeze wire to lift the sailor clear of the water and the craft moves forward, accelerating until the hydrofoils can lift the whole weight. Once on the foils the boat is steered and balanced like a bicycle - two wheels are enough for bicycle stability so two hydrofoils should be fine. Sounds easy?

First prototype

The sail I used with my first prototype was a standard sailboard sail 6.5m2 area but with the mast foot in a slot in the hull rather than being on a universal joint as for a sailboard - I wanted the hydrofoils to stay in a fixed orientation relative to the rig. A forestay was used to support the flimsy sailboard mast against the tension of the trapeze wire and I also experimented with diamond stays to further stiffen the mast. Because of the mast curvature and the constraint from the forestay, it was necessary to allow the mast to move a few mm fore and aft at deck level as the sail was sheeted in and out. To allow this the mast was fitted through a block which could slide in plastic slides mounted in the top of the slot in the hull. The forward hydrofoil was a surface piercing Vee shaped foil and the aft hydrofoil, which also served as a rudder, was an inverted Tee shape. Both hydrofoils were made from glass reinforced plastic (grp) laid up on simple plywood moulds. The angle of attack of the hydrofoils could be adjusted on shore but not while sailing. The hull was made from 4mm plywood and was about 11 feet long and about 270mm square cross section with a punt shaped bow.

Trials with the first prototype

This first prototype was not a great success, but perhaps I just did not try hard enough for long enough. I sailed the boat four times in all and never sailed it with enough wind that the sail could support my weight on the trapeze wire, hence I never really gave it a chance. At that time I had done a little sail-boarding but a water start was beyond my ability so I always attempted to start this craft by up-hauling. I would stand on the hull with the rig lying in the water to leeward. I would then ease out the tackle on the trapeze wire allowing me to lean backwards until the rig came up from the water. A moment later I would find myself lying floating in the water with the craft more or less upright but because the trapeze tackle was let out it would be heeled to leeward by perhaps twenty degrees. If I then sheeted in the craft would start sailing but because of the leeward heel and the drag of my body in the water it would carry huge weather helm which was impossible to steer against and after sailing a few yards it would come head to wind and fall on top of me. Probably I needed to tighten in the trapeze tackle as the craft came upright, or at least before starting to sheet in, but I never managed to adjust the trapeze tackle quickly enough while still keeping hold of both tiller extension and mainsheet. With a lot of practice, an athletic sailor and some improvement in the details of the design I dare say it could be done - perhaps still an interesting project for someone. But after a particularly frustrating swimming session in a muddy creek sometime in late autumn I gave up and dumped the boat at the end of my garden wrapped up in a polythene sheet.

Second prototype

A few years later I renewed my interest in sailing hydrofoils and checked the condition of my first prototype.  Damp had found its way into the polythene cover and the plywood hull was soggy with some of the joints coming unglued. I spent a few weekends attempting to refurbish it using epoxy resin and filler to strengthen and seal all the joints but when I immersed it in the local canal and found that it still leaked like a colander I realised that I was wasting my time with a refurbishment. I then decided to build a new version to a higher standard. Remembering my difficulties with up-haul starts I decided that this time I would fit two small floats each side to give just enough stability for it to float upright at rest. By doing this I was moving away from the original concept of a really portable device you could carry down the beach tucked under an arm, instead it was becoming a small lightweight sailing dinghy.  I suppose that I was also making it into a trimaran, but because the floats were tiny it was really closer in character to a 'winged' monohull dinghy such as the International Moth, albeit with a little buoyancy in the extremity of the wings. I note that some of the recent hydrofoil Moths have inflateable floats in the wings, although I think these are mainly intended for use when learning to handle the craft.

My intention was still to use a trapeze wire to provide lateral righting moment and if I could manage to trapeze with feet on the outriggers rather than the main hull I could gain considerable extra righting moment and hence sail power. Indeed, trapezing properly in this manner the available righting moment should be more than double that possible with a sailboard and with the hoped for low drag lift from the hydrofoils there seemed to be a possibility of really high speed if the ergonomic and control problems could ever be properly sorted out.
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Hull structure

The general arrangement of the hull is shown by the picture of the boat on a car roof rack. The hull and outriggers were built from 4mm plywood mainly joined using fillets of epoxy mixed with microballoon filler. When I started building the boat I thought I was cutting it fine using just 4mm ply but in fact the resulting hull is more than robust enough and I probably should have used even lighter materials. Having said that, there is a limit to how lightly built the outrigger floats could be since these become immersed when the craft is righted from capsize and are then subject to some hydrostatic pressure.


The rig has the general appearance of a sailboard rig but I resisted the temptation to use a standard sailboard rig, even although that would have been a cheap option since I already had a sailboard and a couple of rigs for it. Sailboard rigs are highly developed to be just strong enough for their intended application.  I felt that a boat which was intended to be sailed with a trapeze needed a stronger rig, both to take the localised loading from the trapeze wire and to cope with the heavier aerodynamic loading possible due to the crews weight being applied at greater leverage than for a sailboard.

The mast is a custom made carbon fibre spar which is larger diameter than a sailboard mast and I think has about four times the stiffness of a sailboard mast.  The mast was made with an in-built curve so that despite the stiffness it can still be inserted into the luff pocket of the curved luff sail. The boom is a lightweight half wishbone since it is not required to hold on to it and so a full (double sided) wishbone is not essential. The half wishbone is joined to the mast with a hinge fitting which allows the boom to pivot in the plane of the sail but not to rotate around the mast. This means that the boom is in the same plane as that in which the mast is pre-curved and with a stiff mast I believe that this is one measure which reduces sail twist since the top of the mast does not bend off to leeward.

The sail is fully battened with camber inducers on four of the seven battens. The foot of the mast rotates in bearings moulded into a carbon fiber socket which is bonded into the hull structure.  When the sail is set and the mast fully curved under the normal luff tension the forestay attachment point needs to line up with the axis of the two mast rotation bearings so that the forestay does not slacken as the mast rotates. This is achieved by having a significant kink in the mast just above the top of the mast mounting socket. A further advantage of this is that it places the rotation axis for the whole rig closer to the centre of effort of the rig and that significantly reduces the sheet load. In fact the low sheet loading is one of the better features of this boat.

The mast, mast socket and boom were my first attempt at carbon fibre laminating and it was a bit of a learning curve. I also made the mast rotation bearings and the various fittings in aluminium and titanium but since machining bits of metal was already one of my pastimes, this part of the work was easier for me than the laminating. All the carbon fibre was vacuum bagged and since the components were all long and thin I was able to use lay flat tubing (available from a packaging consumables supplier) for the vacuum bags. But what a lot of different materials and bits and pieces are needed for this kind of job! There is carbon, both cloth and unidirectional, resin with various speed hardeners, peel ply, breather cloth (I used old curtains, probably a false economy), cleaning cloths, parcel tape (some brands are ok with epoxy, others are useless and I don't know how to tell which is which), vacuum bags, release agent, gloves (essential), various solvents, numerous brushes, mixing pots etc. Its a long list of items that need to be purchased and prices tend to be high for small quantities. For a small one off composite construction job it is hard to avoid a lot of wasted left overs - wood work and metal work are both much easier IMHO!

Getting the custom sail made was difficult. The sail maker I first contacted was one with special experience of making sails for International Moth sailing dinghies, which seemed to me a fairly similar type of boat although in those days they had not started to use hydrofoils. Unfortunately my sailmaker failed to deliver the goods and this delayed the project for about a year. After about 6 months he told me for the first time that he was unable to proceed with the job because he could not get camber inducers large enough for my oversized mast section. Perhaps he hoped that would be the last he would see of me but that weekend I took a camber inducer out of my old windsurfing sail and moulded some scaled up copies with glass and epoxy. He did then promise to get on and finish the job in a month. A few months later he still had not started and I then turned to another sailmaker who did make me a reasonable custom sail from start to finish in a fortnight. What is it about sailmakers - I have had really bad experiences trying to get custom sails made by some of them while others have been much better. I certainly won't be going back to the sailmaker who I first approached for my hydrofoil project, nor the one that more recently made new sails for my cruising dinghy. I was rather proud of my hand crafted camber inducers but the new sailmaker rejected them and made his own, telling me that I had copied mine from a somewhat outdated design!


It was my intention to use this project as a test bed to try out various hydrofoil configurations, for example:

  • My original 'Canard' arrangement with surface piercing Vee foil forward and Tee foil aft
  • Surface piercing Vee foils fore and aft
  • 'Wand' incidence control on the forward foil (I was aware of this option from the outset, knowing that it had been tried on powered hydrofoil boats, but the fixed foil arrangements seemed mechanically more straightforward for my first trials)
  • Tee foils that run fully immersed in the lower speed ranges then become planing surfaces at high speed.

Naturally, the first foils I fitted to the boat were the same ones I had made for the first prototype and although I did experiment with some minor modifications to the forward Vee foil I ran out of enthusiasm for the project befor I had got to experiment with the various options above. My original foils, which I still have somewhere in the attic, were moulded from glass reinforced polyester resin with a good proportion of unidirectional glass for bending strength. The forward Vee foil is solid, the strut of the aft tee foil is hollow but the base of the tee is solid. The aft foil is a little bit flexible considering that it carries most of the weight of the boat. When the boat has sailed fully foil borne the steering has felt a bit wobbly and this may be due to the foil strut flexing but I was a bit too preoccupied at those times to look closely to study what is happening. 

Accepted wisdom is that fully immersed Tee shaped hydrofoils offer a better lift to drag ratio than surface piercing Vee shaped foils. This was the main reason that I chose to place the crew weight well aft on this boat, to keep as much of the weight as possible on the supposedly more efficient aft Tee foil.


The boat sailed for distances of up to perhaps 50 to 100 m with the hull fully clear of the water and supported on both hydrofoils. This was achieved in about force 3 wind strength. On one glorious run the boat did stay fully foilborne for longer, crossing most of the width of Grafam water (a few hundred meters) before I had let it come down to keep clear of the shallow water at the shore. Once the hull was completely clear of the water there was very little spray or wake. If you are wondering why there is no photograph to show this it is because there was no camera in the right place at the right time. I carried out the experiments with no assistance from any support boat and this was before the advent of waterproof digital cameras so it was not quite so easy to photograph such experiments while working single handed.

Although the boat had been designed for use with a trapeze wire it was never actually sailed in that mode, I realised that I just did not have the necessary skill. With hindsight I should have practiced with a more conventional trapeze dinghy, then tried the technique on the hydrofoil.  Rather than use the trapeze on my boat I just sat on the outrigger, which was uncomfortable due to lack of leg room. I had made the beam accross the outriggers rather short, considerably less than the overal beam of a Moth dinghy. The rather narrow overal beam was mainly to allow the boat to be carried on a car roof rack without the width exceeding the width across the wing mirrors and in any case I didnt expect to need a lot of overal beam if trapezing. Sailing the boat sitting on an outrigger, the bending moment at the mast base and the load on the mast rotation bearings was far higher than it would have been with the mast supported by a trapeze wire, but the mast and bearings stood up to this, perhaps my novice carbon fibre work was not so bad after all. The mast bearings, which incorporated UWMWP sleeves remained free turning despite the high loading.

I did take the boat to the Weymouth speed trials one year and I made two timed runs of the 500m course at about 9 knots averaged over the 500m. The wind at the time was too strong for my skill level and so the boat was never foil borne during these runs, indeed at times it was almost stationary as I struggled to recover from near capsizes, so the peak speed would have been well above the average. Also, at that time I had recently modified the forward Vee foil, truncating the bottom of the Vee to try to improve low speed tacking, but that also meant that the Vee foil could no longer lift the bow high enough to get enough incidence on the aft foil to achieve full take off, hence my two runs at Weymouth were with the craft 'foil assisted' rather than fully foiling. I would add that pretty well everyone I talked to at the speed trials told me that I had built a ridiculous boat and that there was no way one  could ever balance on just two hydrofoils - obviously I needed to start again with a proper hydrofoil with at least three foils to stabilise it against sail heeling force. What people didnt realise was that lateral heel control was never the problem, with an easily sheeted rig it is quite easy to just lean on the wind and control your angle of heel with the mast inclined to windward so that the weight of the whole boat is offset to windward of the hydrofoils, this adding considerably to the available righting moment.


The reader will not be surprised that this boat was easy to capsize. Fortunately it was also an easy boat to right from a capsize. When the boat is capsized with the crew not on board it floats on one of the small side floats together with the forward part of the central hull with the buoyancy of the mast preventing complete inversion. The first action of the crew is to pull the main hull down to the water immersing the lower float. It is then possible to place a foot on the bow of the immersed float to help climb up over the underside of the main hull to scramble aboard just forward of the forward float beam. Sometimes the boat comes upright during this process, if not one can just sit on the main hull and lean back holding onto the upper float to right the boat.

One difficulty is that if the crew becomes separated from the boat during capsize the boat can drift away from the crew faster than it is possible to swim. A heavier sailing dinghy probably drifts more slowly when capsized and a sailboard certainly drifts more slowly since it has very little air drag once the sail is lying flat.  I mostly sailed the boat in winds of force 2 to 3 and under those conditions I could catch up with the drifting boat swimming with the hindrance of a wet suit and life jacket. But on one occasion I sailed the boat in a stronger wind, probably a force 4 and found that I was unable to get back to the boat after capsising. I was eventually rescued by a sailing club safety boat but it was about the most scary incident I have ever had when sailing.

Low speed manoeuvring

The boat was always difficult to steer at low speed and tended to get caught in irons when tacking which could be most frustrating. The rudder surface, which is the strut of the aft hydrofoil, is of small area to minimise drag and is only marginally large enough when sailing slowly. Since the sail came right down to deck level I had to move right aft to get past the sail when tacking and that deeply immersed the stern. This had the advantage that it forces the floats into the water giving much needed lateral stability but the deeply immersed stern also resisted being steered sideways by the small rudder surface. Also the rig may have been mounted too far aft for easy bearing away after a tack. I positioned the rig well aft primarily to avoid the trapeze wire pulling the crew forward when trapezing near the stern. The forward hydrofoil as originally constructed was a Vee shape with a small vertical fin protruding from the bottom of the Vee. This fin was later cut off and the bottom of the vee truncated to a flat. This reduced the depth of the forward hydrofoil which did help to get the bow through the wind when tacking, but as discussed above it also prevented the foil from lifting the bows high enough to take off, so in its final configuration (i.e. present configuration since I still have the boat, although I havent sailed it for many years) the boat is probably not capable of becoming fully foil borne. 

I suspect that front steering, that is steering acting on the forward rather than aft hydrofoil, would improve low speed maneuverability of this boat - the lightly loaded bows would push sideways much more easily than the deeply immersed stern of the boat. However, a forward steering hydrofoil is mechanically more complicated and from the outset I was trying to keep things as simple as possible. 

Further development

Obviously other people have now taken my 'bifoiler' concept to a far higher level of refinement than I achieved with my early experiments and realistically there is not much chance of my catching up at this stage, even if I wanted to. In any case, this kind of thing is not my main interest in sailing, as can probably be seen from the rest of this website. My main interest in sailing has always been cruising to interesting places, mostly in sailing dinghies, although in the last couple of years I have been toying with the idea of building a larger cruising boat, possibly a catamaran.

The most obvious difference between my hydrofoil boats and the modern bifoilers is that nearly all the modern boats use a 'wand' system to control the height of the boat above the water. This does work remarkably well. When I built my boats I was aware of this possibility since some of the early power driven hydrofoils (e.g. those by Hook) used mechanical 'feelers' to sense the height of the boat above the water. However, inspired by the success of the simple sailboard, I was always trying to keep my designs really simple and a wand system looked like greater mechanical complexity. In a way, the canard foil arrangment that I used is not so different to the wand system in that the forward surface piercing foil acts as a large 'wand' and by tilting the whole hull it controls the angle of incidence on the main lifting foil which is mounted further aft - there just isn't any more foils further aft of that, just the wand and the main foil!

I am aware that a few other people have experimented with canard bifoilers with surface piercing forward foils, one of these being the renowned naval architect and racing dinghy designer, the late Frank Bethwaite. During the last few years that he was able to go sailing, Frank Bethwaite experimented with canard bifoilers using the rigs and hulls as for the 49'er racing dinghy that he designed and which is used for the sailing Olympic games. Frank's bifoilers did use bow steering. I have seen pictures showing that these boats certainly got foilborne but I dont think they achieved the performance of modern wand controlled boats. Having said that, I think Frank had to retire from sailing before he had fully developed these designs, so perhaps there is still scope for further work in this area.

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I think this picture suggests how the boat is handicapped by not using the trapeze - there should be a lot more potential if the crew weight were say twice as far off the centreline